Oligodendrocyte precursors (OPCs) differentiate into oligodendrocytes that are the myelinating cells of the vertebrate CNS. Myelin sheaths wrap axons in the brain and spinal cord and maintain axonal function and promote rapid conduction of electrical impulses. Any damage to myelin sheaths, such as occurs in multiple sclerosis, results in loss of axonal conduction and ultimately axonal degeneration and irreversible neural disability leading to serious physical or mental impairments. Multiple sclerosis is a devastating disease that affects more than 300,000 individuals in the United States. Current therapies are directed towards regulating the inflammatory aspects of the disease, however long term functional recovery will depend upon successful myelin repair in the CNS. Recent studies suggest that many areas of demyelination in the brains of MS patients contain OPCs but the ability of these cells to repair damage is limited because they fail to differentiate for reasons tat are currently unknown. A detailed understanding of the mechanisms controlling OPC maturation and myelination will therefore provide new insights and novel therapeutic strategies for enhancing myelin repair in MS. The experiments described in this proposal will explore the roles of the intracellular signaling molecule cyclin dependent kinase 5 (Cdk5) and its co-activators p35/p39, in regulating the development of OPCs, myelination and remyelination. Cdk5 is known to be involved in various signaling pathways that are key for CNS development. Our preliminary data has revealed novel functions of Cdk5 in controlling the development of OPC and myelination. The proposed study will explore whether Cdk5 within cells of the oligodendrocyte lineage regulates their development and myelination in vitro and in vivo using molecular and genetic approaches. We will identify the roles p35/p39 play in mediating Cdk5 modulation of OPC maturation and myelination. To determine whether the Cdk5 pathway is a novel potential target for therapeutic development we will test whether Cdk5 is essential for remyelination in adult CNS after the induction of focal demyelinating lesions. To accomplish this we will selectively delete Cdk5 from OPCs in the adult CNS during demyelination. Successful completion of the proposed studies will provide critical insights into the signaling mechanisms regulating OPC maturation and myelination and provide novel targets to promote myelin repair.